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Oversized pistons will indicate the overbore with the amount of overbore, as in .020 lightly stamped into the top of the piston.
The reason for the compression test with the engine warmed up is to allow the cylinders to be lubricated and any imperfections which could alter the sealing with a cold engine over a warm engine because it doesn't matter what the compression is when the engine is cold, it is what it is when the engine is warm. A really well assembled engine may not have much difference, but a worn engine could have greater than the magical ten percent difference between cylinders, so remember, we don't drive around with cold engines anyway, so, if greater than ten percent cold, warm the engine up and check again, and if it is the same, then you can know for sure there is a compression or sealing problem.
The reason for the compression test with the engine warmed up is to allow the cylinders to be lubricated and any imperfections which could alter the sealing with a cold engine over a warm engine because it doesn't matter what the compression is when the engine is cold, it is what it is when the engine is warm. A really well assembled engine may not have much difference, but a worn engine could have greater than the magical ten percent difference between cylinders, so remember, we don't drive around with cold engines anyway, so, if greater than ten percent cold, warm the engine up and check again, and if it is the same, then you can know for sure there is a compression or sealing problem.